The present invention is directed, in general, to detecting defects in integrated circuits and, more specifically, to detecting surface damage or other irregularities at minute levels on the surface of capacitive sensor integrated circuits.
Unlike typical integrated circuits which are completely encapsulated in protective packaging except for conductive leads or pins, fingerprint sensors and other capacitive integrated circuits must necessarily include an exposed sensor surface proximate to the integrated circuit core. A simplistic example is illustrated in FIGS. 3A and 3B. FIG. 3A is a plan view of a capacitive fingerprint sensor 300 showing the layout of capacitive sensor electrodes without overlying dielectric or passivation layers; FIG. 3B is a cross-sectional view of the capacitive fingerprint sensor 300 taken at section line Axe2x80x94A, showing the overlying layers.
Capacitive fingerprint sensor 300 includes a two-dimensional array of pixels or cells each containing, in the example shown, a pair of capacitive sensor electrodes 301 and 302, formed by patterned conductive plates over a substrate containing devices (not shown) implementing the detection and control circuitry. An interlevel dielectric 304 (e.g., an oxide) is disposed over and between the capacitive sensor electrodes 301 and 302. A passivation layer 305 is disposed over the interlevel dielectric 304 and forms a sensor surface 306 on which the finger is placed for sensing of the fingerprint pattern.
A problem with capacitive fingerprint sensors of the type described is localized surface damage 307 (e.g., scallops or troughs) and other irregularities in the sensing surface 306 of the top passivation layer 305. At the levels of 10 to 40 microns (xcexcm) in length and less than 1 xcexcm wide, such surface irregularities are virtually impossible to detect with microscopic inspection. With such light damage, the fingerprint can still initially be detected by the affected pixels and the damage is not visible in the fingerprint image.
During use, however, the damage site degrades over time, modulating the pixel and changing the image. The pixel gain may degrade, for example, to approximately one third of the original gain, behaving as if coupled to a floating capacitive plate located in the region of the surface damage. The surface damage or irregularity thus manifests after roughly one to three months of use as a xe2x80x9cwhitexe2x80x9d region in the fingerprint image (i.e., a dead or largely insensitive region within the sensor array) resulting from an increase in the interference between coupling of the capacitor plates. Therefore, fingerprint sensor devices initially functioning properly become functionally unacceptable after only a relatively brief period of use, without knowledge of (or any means for detecting) the surface damage or irregularity because no method of screening the problem was known.
There is clearly a need in the art for a method of detecting minute levels of surface damage and/or irregularity above the sensor capacitors of a fingerprint sensor device or other coupled fringe capacitor circuit.
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in integrated circuits, a technique for detecting minute surface damage and irregularities on the sensing surface of a capacitive integrated circuit by acquiring a preliminary image of the capacitance readings for the sensor array, coating the sensing surface with an electrolyte solution, then acquiring an additional image while the sensing surface is coated and/or after the electrolyte solution is removed. The electrolyte solution accelerates manifestation of pixel degradation and failure caused by surface damage or irregularities. Defective regions are identified by change of grayscale pixels in the preliminary image while the electrolyte coating is on the sensing surface and then again after the electrolyte coating is removed.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art will appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art will also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words or phrases used throughout this patent document: the terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprise,xe2x80x9d as well as derivatives thereof, mean inclusion without limitation; the term xe2x80x9corxe2x80x9d is inclusive, meaning and/or; the phrases xe2x80x9cassociated withxe2x80x9d and xe2x80x9cassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term xe2x80x9ccontrollerxe2x80x9d means any device, system or part thereof that controls at least one operation, whether such a device is implemented in hardware, firmware, software or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, and those of ordinary skill in the art will understand that such definitions apply in many, if not most, instances to prior as well as future uses of such defined words and phrases.